1. Field of the Invention
The present invention relates to a power generator, a timepiece and electronic device having the power generator and a cogging torque adjustment method for the power generator, the power generator being adapted for supplying power in an electronic clock etc. More specifically, it relates to a technique for optimizing a cogging torque (non-excitation torque/detent torque of a step motor) of a power generator.
2. Description of the Related Art
As shown in FIG. 1, an electronic clock with a crystal oscillator as a time standard has a power supply 10 having a small-size power generator 20 and a secondary power supply 30. The power supply 10 actuates a step motor etc. of a processor 14. As shown in FIG. 2, the small-size power generator 20 is provided with a rotor 21 to be rotated by a transmitted rotary drive force, a stator 22 sandwiching the rotor 21 and a power-generating coil 23 wound around a magnetic core constituting a magnetic circuit together with the stator 22 and the rotor 21. The rotor 21 has a power-generating gear train 60 for speeding up and transmitting a rotation of an oscillating weight 25.
In order for the rotor 21 to remain at a desired position when no load is applied, outer notches 221 and 222 for the magnetic saturation portion on a periphery of the rotor 21 are formed on the stator 22 as shown in FIG. 14. The rotor 21 is a permanent magnet having N and S magnetic poles. When the rotor 21 remains at a certain angular position and the rotation of the oscillating weight 25 is transmitted through the power-generating gear train 60, the magnetic poles N and S are rotated to generate electromotive force to the power-generating coil 23. Since a cogging torque is applied to the rotor 21, the rotor 21 is biased to remain at a predetermined angular position (rotor stop position without applying load—referred to “no-load rotor stop position” hereinafter.).
Accordingly, in order to rotate rotor 21 the oscillating weight 25 has to be capable of transmitting greater torque to the rotor 21 than the cogging torque.
However, the size and thickness of respective components of an electronic clock have been reduced to minimize its overall thickness. Thus, the size and weight of the oscillating weight 25 of the small-size power generator 20 are necessarily reduced. Accordingly, with the conventional small-size power generator 20, when the size and weight of the oscillating weight 25 are reduced while the magnitude of the cogging torque applied to the rotor 21 does not change, rotation of the oscillating weight 25 can be hampered, thus rendering it incapable of charging the secondary power supply 30.
In another type of power generator, the rotor of the power generator is rotated by a mechanical energy source such as a power spring. However, when the size of the power spring etc. is reduced, the rotation of the rotor can be hampered, thus causing the same problem in charging the secondary power supply.
Accordingly, it has been desired that the magnitude of cogging torque is made as small as possible so as to facilitate rotation of the rotor even when the size of the oscillating weight 25 and the power spring etc. is reduced.